With the recent popularization of smart phones or the like, there is an increasing demand for high-speed wireless transfer. In the Third Generation Partnership Project (3GPP) that is one among standardization organizations, standardization for Long Term Evolution (LTE) has been performed. Currently, Release 11 (Rel-11) standardization has almost been finished, and Rel-12 standardization has been performed.
For downlink in LTE, as modulation schemes, Quadrature Phase Shift Keying (QPSK), Quadrature Amplitude Modulation (16 QAM), and 64 QAM are supported. While only two bits can be transmitted on one modulation symbol with QPSK, 4 bits can be transmitted with 16 QAM and 6 bits can be transmitted with 64 QAM. More precisely, 16 QAM has higher frequency efficiency than QPSK, and 64 QAM has higher frequency efficiency than 16 QAM. However, when a channel state is poor for a base station apparatus (an evolved Node B(eNB)) and a terminal apparatus (User Equipment (UE)), the greater the number of transmission bits on one modulation symbol, the more likely a bit error is to occur. Accordingly, in LTE, a technology is employed in which a modulation scheme is adaptively selected depending on a channel state between the eNB and the UE and which is referred to as an adaptive modulation. Moreover, in addition to the modulation scheme, a coding rate of an error correction code is also adaptively changed in LTE. For example, in LTE Frequency Division Duplex (FDD), the UE estimates a downlink channel state based on a reference signal that is transmitted by the base station apparatus, and notifies the eNB of Channel Quality Information (CQI) that is obtained. The eNB selects a modulation scheme that has the highest frequency efficiency from among combinations of the modulation schemes and the coding rates (Modulation and Coding Schemes (MCSs)) that have a prescribed error rate or less, using the notified CQI, and performs downlink transfer using the selected MCS. In this LTE, adaptive selection of the MCS depending on the channel state can realize high throughput.
Moreover, in standards up to and including the LTE Rel-11 standards, a technology that is referred to as Almost Blank Subframes (ABSs) is introduced. In this technology, a certain base station apparatus does not perform data transmission and the like on a prescribed subframe, or determines that the transmission has to be performed with a lowered power, and notifies neighbor base station apparatuses of information indicating the prescribed subframe. The neighbor base station apparatuses can perform downlink data transfer on the terminal apparatus that is under the control on a subframe on which a small amount of interference from a certain base station apparatus occurs.
Furthermore, in the Rel-12 standards, it is considered that in addition to the eNB in the related art, a pico base station (is referred to as a small cell) is arranged within a cell that is covered by the eNB. Moreover, the pico base station does not necessarily need to be equipped with a function as the base station, and may be configured as a forward-extending antenna (Remote Radio Head (RRH)). Because it is presupposed that multiple pico base stations are arranged within a cell and that sectoring is not performed, it is assumed that because interference between sectors does not occur and the like or for other reasons, a likelihood that a high Signal to Interference plus Noise power Ratio (SINR) will be obtained is higher since Rel-8. Accordingly, in 3GPP, the introduction of 256 QAM that enables 8-bit transmission on one symbol has been considered in addition to the introduction of QPSK, 16 QAM, and 64 QAM. With the introduction of 256 QAM, the UE that can receive data at a high SINR can further increase the throughput.
Incidentally, a value in a case where modulation schemes up to and including 64 QAM are assumed is defined for the CQI that the UE notifies the eNB (or the pico base station) of. For this reason, although the UE notifies the eNB of the greatest CQI, the eNB determines that the channel state is poor for transmitting data to the UE with 256 QAM, and regardless of an environment in which even though the transmission with 256 QAM is performed, the transfer can be performed without any error, the eNB is expected to transfer data using 64 QAM for the downlink. Furthermore, a channel that is referred to as a Physical Downlink Shared CHannel (PDSCH) is used for the data transfer for the downlink, but the MCS that is used on the PDSCH is notified using a channel that is referred to as a Physical Downlink Control CHannel (PDCCH), for transmission of control information. Nevertheless, because 256 QAM is not defined in the standards up to and including the Rel-11 standards, 256 QAM is difficult to configure on the PDCCH.
In this manner, with the CQIs and MCSs up to and including those in the Rel-11, it is difficult for the UE to make a request to the eNB for 256 QAM or it is difficult for the eNB to notify the UE that transfer with 256 QAM is performed. Accordingly, as disclosed in NPL 1 and NPL 2, in the standards up to and including the Rel-11 standards, the CQI and the MCS are defined with 4 bits and 5 bits, respectively, but because 256 QAM is supported, it is considered that an amount of information is increased by one bit for each of the CQI and the MCS, and thus, the CQI and the MCS are defined with 5 bits and 6 bits, respectively. Nevertheless, when the number of bits is increased, problems occur not only in that an increase in the control information decreases downlink throughput, but also in that an increase in the number of bits of the control information brings about the need to change a mechanism that is referred to as blind decoding that is performed on the control information.
Accordingly, in current 3GPP, as disclosed in NPL 3, it is proposed that a mode (hereinafter, referred to as a 64 QAM mode) in the related art in which a CQI index that is calculated with a CQI table which corresponds to modulation schemes up to and including 64 QAM is notified, and an MCS index is notified with an MCS table that corresponds to modulation schemes up to and including 64 QAM, and a new mode (hereinafter, referred to as a 256 QAM mode) in which a CQI index that is calculated with a CQI table which corresponds to modulation schemes up to and including 256 QAM is notified, and an MCS index is notified with an MCS table that corresponds to modulation schemes up to and including 256 QAM are prepared. Furthermore, in NPL 4, it is proposed that which table is selected is determined by a notification of a higher layer that is referred to as Radio Resource Control (RRC). With switching between the modes with the RRC, the transfer with 256 QAM can be supported without changing the number of bits of the CQI and the number of bits of the MCS.